In the design of electric machines, it is known to select structural parameters such as slot number depending on the intended application and desired performance characteristics of the machine. However, not all values of the structural parameters are used in practice. There is room for improved performance of electric machines, particularly in robotics.
Electric machines typically use electrically conductive wire turns wrapped around soft magnetic stator posts (teeth) to generate flux. The manufacturing process for this type of motor construction can be time consuming and expensive. As well, such motors typically have a torque to mass ratio that makes them relatively heavy for mobile actuator applications such as in robotics where the weight of a downstream actuator must be supported and accelerated by an upstream actuator.
Common permanent magnet direct drive motors can be difficult to assembly because of high permanent magnet forces between the rotor and stator. These high magnetic forces typically require complex fixtures for assembly to avoid damage to parts and injury to personnel as the rotor and stator are brought together.
Large diameter, low profile bearings that are used in many motion control devices such as robot arm joints, must typically be physically retained in the housings to prevent separation of the bearing assembly. Many low profile bearings also tend to be relatively low tolerance compared to larger profile, smaller diameter bearings. Moreover, bearings typically require an adjustable preload that is typically provided by a threaded or other type of member. This is difficult to fit into a low profile assembly and is especially challenging with thin section bearings.
In a common axial flux actuator, the bearings are located at the inner diameter of the magnetic active section of the rotor. This setup is a common practice because placing a bearing at the outer diameter of the rotor induces more drag and the overall bearing profile increases as the bearing diameter increases. Bearings on the OD of the rotor will also tend to limit the rotational speed of the device.
To make a single inner bearing work with a single rotor/single stator, either the rotor and stator structures must be thickened to provide a stiffer structure to reduce deflection, or the air gap distance must be increased to accommodate the rotor and stator deflection. The first method results in a heavier device and larger envelope which reduces actuator acceleration and torque density. The latter method result in a reduction of torque due to the larger air gap distance.